OTTAWA -- Prime Minister Justin Trudeau said Canadians "stand with" the United Kingdom as its Prime Minister Boris Johnson was admitted to the ICU with worsening COVID-19 symptoms.

"My thoughts are with Prime Minister Johnson and his family during this time. I've worked with Boris for some time now, and I know how committed he is to serving the people of the U.K. He's a fighter, and he’ll be fighting now to regain his health and get back to the job he loves," Trudeau said, speaking from the front steps of Rideau Cottage on Tuesday.

"Sophie and I and all Canadians are wishing him a speedy recovery. To our British friends, Canadians stand with you. We will get through this together."

The 55-year-old was admitted to the ICU at St. Thomas' Hospital in London on Sunday due to a fever and cough that persisted 10 days after he was diagnosed with COVID-19.

"The prime minister has been stable overnight and remains in good spirits. He is receiving standard oxygen treatment and is breathing without any other assistance," Johnson's spokesman, James Slack, told the Associated Press.

Slack added that Johnson was not put on a ventilator but would not provide details on the kind of oxygen treatment the prime minister was receiving.

According to infectious disease specialist Dr. Abdu Sharkawy, it could be worrying if Johnson is put on six liters or more of oxygen.

"We know from our own experience that when patients reach a threshold of about six litres per minute in need, we often preemptively intubate them because we can predict that they may deteriorate fairly rapidly thereafter if they're not put on a ventilator," said Sharkawy.

However, Sharkawy said it's important not to speculate about Johnson’s condition, as experts "may be having him looked after in the ICU out of really nothing more but an abundance of caution."

Speaking to CTV Power Play Host Evan Solomon on Tuesday, British High Commissioner Susan le Jeune d'Allegeershecque said Johnson is "obviously not very well, but he’s stable so he’s not got any worse."

"They will keep him there for as long as he needs that level of attention and care, and I think we're all hoping it’ll be as short a period as possible. But obviously he has to stay there until his condition is significantly improved," she said.

While Johnson remains in the ICU, his Foreign Secretary Dominic Raab has temporarily taken over many of Johnson's duties.

According to the Associated Press, as of Monday 51,608 people in Britain have been diagnosed with COVID-19, and 5,373 have died.